Polymeric foams exist in the form of polyurethane or epoxy foams in which resins are foamed and cured into solid materials. Polyurethane foams are established materials with many reports on their formulations and processing requirements. Polyurethanes are commonly foamed using an in-situ chemical side reaction that produces gaseous CO2 during the cure reaction. This reaction is specific to the properties of isocyanates, a key molecular building block of polyurethanes, which form carbamic acids in the presence of water and cleave off CO2.
Considerably less knowledge exists regarding how to produce epoxy-based foams, mainly because simple chemically compatible foaming reactions as available for polyurethane foams do not exist for epoxy-based foams. Therefore, foamed epoxies are normally produced via addition of physical blowing agents, such as inorganic compounds that thermally decompose at low temperatures and produce gases, or via addition of low volatile liquids, such as freons or similar fluorinert compounds, that also result in gas formation and expansion during the exothermic epoxy cure. See, e.g., U.S. Pat. No. 6,110,982 to Russick et al. However, a process for foaming curable epoxy resins via the generation of CO2 from a suitable precursor (e.g., maleic anhydride) and curing into solid foams was recently described in U.S. Pat. No. 8,003,730 to Celina.
However, detailed methods on how to chemically foam and concurrently cure cyanate esters into solid foamed materials do not exist. Major challenges are found in the selection of a suitable base resin, availability of suitable cure reaction, compatibility with a chemical foaming reaction, and sufficient early foam stability (rapid viscosity increase) during the transition from a liquid resin to a curing foamed material. The final cure reactions must retain the solid foam structure and yield good conversion of the reactive pre-cursors.
Whether polyurethane or epoxy foams, such materials will decompose when exposed to temperatures above 300-400° C. because of their intrinsic molecular building blocks. The key constituents, such as many epoxy resins, amine curatives, and polyols, are thermally weak and degrade via chain scission into low molecular weight fragments. Under oxidative conditions the materials will start to burn, and under inert (nitrogen atmosphere) conditions, the materials will pyrolize and decompose into gaseous, liquid and some carbonaceous residues. Char residues of urethane and epoxy foams at temperature above 400° C. are often in the range of only 10-20%. There are currently no commercial polymeric foams available that offer intrinsic high levels of solid char residues coupled with good retention of dimensional features at pyrolysis decomposition temperatures.
Therefore, a need remains for a cyanate ester foam that can be charred and yet retain a foam structure.